Undeveloped (ie. mining, colony, and outer) planets may have up to 1 squadron of fighters assigned to each. A squadron consists of 12 fighters, 18 for Hydrans.
Populated planets may have up to 2 squadrons.
Major planets may have up to 3 squadrons.
Small space-based stations (CPLs, SAMs, etc.) may have up to 2 hangar modules worth of fighters each. If orbiting a planet (or on the same tactical map), these fighters count towards the planetary limit.
Monitors (once developed) may be assigned to planets at no Command Cost. No planet may have more than one Monitor at a time.
Planetary defenses may not operate more than 35 sectors away from their planet. Weapons may still be fired at units farther away.
If a permanent base is orbiting a planet, ignore all above restrictions but all planetary defenses must fit within the base's CR, including fighters and Monitors.